Unlike SopPlayer, SopCast WebPlayer can not run solely. It's just an ActiveX plugin for web browser, you have to embed it on your web page to play channels of SopCast. WebPlayer can be controled with javascript code.

The plugin mozactivex-ff-15.xpi is for firefox version
1.5.0.x. But it works for 2.x version also, please have a try.

4.2 Install the Plugin

Drag and Drop the file mozactivex-ff-15.xpi into the firefox's
windows, a installation window will popup. Click
button "Install Now" to install the plugin.

4.3 Config the Plugin

Config the plugin by enter a URL: about:config in address box of
firefox, some configurations will be displayed in text window.
Select the security.classID.allowByDefault and set the value to
true.

4.4 Restart Firefox

Restart the firefox and visit http://www.sopcast.com/player/. If you have installed the SopCast WebPlayer, you can see the
WebPlayer's user interface. If you have never downloaded and
installed it, download it here , install it and refresh the page.